December 11, 2013

The Curiosity mission has achieved another milestone as scientists have determined that the rocks inside Gale Crater that were analyzed by the rover are very old – even on geologic time scales, but were exposed very recently. The achievement of utilizing in-situ age-dating methods using radiogenic and cosmogenic noble gases marks a first in planetary exploration.

The SAM instrument performed a number of experiment runs using different settings, CheMin conducted multiple analyses of the sample and the APXS instrument was used to analyze the drilled sample that was distributed to the Observation Tray of the rover. All these measurements were combined to determine the age of the rock.

The K-Ar age-dating method is commonly used on Earth to determine the age of rock samples, but had never been used in space before. MSL used its APXS instrument to determine the elemental composition of the Cumberland rock. Cumberland is rich in potassium and thus includes potassium-40 which is a potassium isotope that spontaneously decays and produces argon-40 with a half life of 1.25 billion years.

In its initial state, the rock started out with no argon-40 (or a negligible abundance of argon-40). Over the evolution of the Cumberland rock, potassium-40 decayed and argon-40 was formed that accumulated in the material. Heating the rock allows the argon-40 to diffuse out.

When being delivered to the SAM oven, the sample was heated and the exact amount of argon-40 that was released was measured by the Quadrupole Mass Spectrometer of the instrument. Knowing the potassium abundance of the rock, the mass of the heated sample and the amount of argon-40 that is released allows a fairly accurate calculation of the age of the material. Unfortunately, the SAM instrument can not measure the mass of a sample inside one of its sample cups and neither can the sample handling system of the rover. Ground testing has been used to determine the average sample mass that enters the SAM instrument, but that obviously comes with a relatively large error bar. It has been determined that SAM receives samples of 135 +/- 18 milligrams. This error bar can not be reduced which is the limiting factor for age-dating using the SAM instrument.

Nevertheless, the science team was able to determine that the Cumberland rock formed 3.86 to 4.56 billion years ago. This figure is in good agreement with the previous estimate of 3.6 to 4.1 billion years based on crater density measurements.

Photo: NASA/JPL/Caltech/MSSS

Cumberland Drill Hole

Previously, data from Curiosity was used to confirm that the material was of igneous origin. It is likely that the material formed on or outside the rim of Gale Crater and was transported to its current location by sedimentary processes, being washed down before being deposited inside Yellowknife Bay and forming the geological unit that has become known as Sheepbed.

Sediment transport at Gale Crater

Image: NASA/JPL/Caltech

The second part of the age-dating results presented in the paper is that the Cumberland site was just recently exposed (on a geologic time scale). Measuring the abundance of cosmogenic isotopes, scientists were able to pin-point how long ago the Cumberland site became exposed.

Cosmogenic isotopes are rare isotopes that are formed when a high-energy cosmic ray impacts with matter. As a result, a large number of nucleons are expelled from the object hit. This cosmic ray spallation creates rare isotopes that are not a part of the material in its original state.

On Mars, cosmic rays can penetrate as deep as two to three meters and create cosmogenic isotopes. Knowing the Galactic Cosmic Ray Dose at the surface and sub-surface and with that the formation rate of cosmogenic isotopes, allows scientists to deduce the amount of time a sample spent at or near the surface by measuring the abundance of a number of cosmogenic isotopes.

For the Cumberland sample the following isotopes were measured by SAM: argon-36, neon-21, and helium-3. As the samples were heated up, the isotopes were released as gases and the Quadrupole Mass Spectrometer precisely measured the individual abundance of the three gases.

Argon-36 is generated through the capture of thermal neutrons by Chlorine while neon-21 is produced from spallation of Mg, Si and Al. Helium-3 is formed by spallation of O, Si and Mg. The depth-dependence curves of those three isotopes is also well understood – the two spallation isotopes have a maximum of production rate at a depth of 15 centimeters followed by an exponential decline. Argon-36 has a larger sub-surface maximum at a depth of about 60 centimeters. The depth-dependences are due to different formation processes and the nuclear cascade occurring in the uppermost rock regions.

To take advantage of the different production rates at the different depths for all three isotopes, the ratios of Ar/He, Ar/Ne and He/Ne are calculated. These ratios can be used to put together the story of how the rock became exposed, whether it occurred instantaneously or if a steady erosion caused a progressive downward migration of the surface. (Instantaneously refers to geological time scales.)

Photo: NASA/JPL/Caltech/MSSS

The Ar/He and Ar/Ne ratios are used to learn about the exposure time scale while the He/Ne ratio provides a cross-check as it was expected to be 8 for both scenarios (instantaneous exposure or steady erosion).

The Ar/He ratio was measured at 1.7 +/-0.5, the Ar/Ne ratio was 12 +/-5 and the He/Ne ratio was 7.5 +/-2.6 which is close to the expected number. The determined Ar/He and Ar/Ne ratios correspond well to the predicted ratios of the no-erosion scenario of 1.5 and 13. Additionally, the measured ratios are very different from those calculated for the steady erosion scenario.

Using the cosmogenic isotopes, scientists concluded that the rock was exposed approximately 80 million years ago which is very recent. It is also possible that the figure represents the sum of multiple periods the rock spent on the surface, but that is highly unlikely.

Knowing that the rock was formed about 4 billion years ago and became exposed at the surface 80 million years ago, scientists also identified the mechanism that moved the rock to the surface. The Sheepbed layer is exposed by a quick erosion process that occurs between the different surface layers. Cumberland is part of the Sheepbed Unit that is located below the Gillespie layer and the Point Lake layer. Looking at images taken by Curiosity at Yellowknife Bay, it is obvious that the different geological layers erode at different speeds.

The weak mudstone of the Sheepbed unit is eroded very easily as it is slowly removed by Martian winds and dust. As Sheepbed is sandblasted by wind scouring, the harder layers such as Gillespie are undercut. As the scarp retreats, the Sheepbed unit becomes exposed. This erosion is quite fast and progresses at an average rate of about one meter per million years.

Assuming the scarp retreat is an ongoing and persistent process, it is very easy to find freshly exposed surface by locating the base of the downwind scarp. This will play a major role in the selection of future drill targets as teams seek to find out whether organic chemicals have been preserved in rocks. As rock is exposed at the surface, cosmic radiation causes the destruction of organic molecules. That is why teams search for freshly exposed material that did not experience the harsh surface radiation environment for long periods of time.

Future Drill Site Selection

The Yellowknife Bay site (left) and the KMS-9 site at Waypoint 4 (right) show similar signs of active scarp retreat.

MSL APXS puts recent Rocknest Soil Samples into Global Context

December 4, 2012

Image: NASA/JPL-Caltech/University of Guelph

This graph released by the Mars Science Laboratory Mission puts the soil recently analyzed by the Curiosity Rover in a global context using APXS (Alpha Particle X-Ray Spectrometer) data from MSL and the two Mars Exploration Rovers, Spirit and Opportunity. For more about MSL APXS and its objectives, visit the instrument overview site.

Curiosity spent several weeks in October and November 2012 at a site called Rocknest which is a drift of wind-deposited soil about 2.5 by 5 meters in size. There, the rover performed a wheel scuff to expose surface material which was subsequently analyzed by APXS that was used for several integrations at Rocknest. Data from the wheel scuff mark, called ‘Portage,’ is shown in blue.

The data from the Mars Exploration Rovers (red and green) are from several batches of soil, while MSL data is based on one APXS integration only. These three different data sets represent data points on a global scale on Mars as the three rovers operated in different areas of the planet: Curiosity landed inside Gale Crater in August, Spirit has completed its mission at Gusev Crater and Opportunity is still busy at Meridiani Planum.

It is clearly visible that the samples investigated by the three rovers are very similar in composition. Martian dust is transported throughout the Martian Atmosphere by storms on a regional and global scale so that the composition of dust at any given location on Mars is similar. Also, this shows that the soil that was examined by Curiosity was indeed just ‘ordinary’ Martian dust, which puts the measurements made by the two Laboratory Instruments SAM and CheMin into context.

“It’s a significant technological and analytical achievement,” said Iain Campbell, a member of the University of Guelph research group involved with the MSL mission.

The graph includes error bars that indicate the variations for the given number of soils measured by MER. The concentrations of silicon dioxide and iron oxide were divided by 10 while the concentrations of nickel, zinc and bromine were multiplied by 100 to fit into a single chart.

Photo: NASA/JPL/Caltech

MSL Scuff Mark at Rocknest

Photo: NASA/JPL/Caltech/MSSS

APXS on Mars as seen by MSL's Mast Camera

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